Species, endowed with an open-ended capacity for learning, which is one of the highest evolutionary achievements,will profit most from this ability, if they are urged one way or other to invest any surplus of energy in expanding and refining their behavioural repertoire and in adapting it to prevailing circumstances, while incurring as little risk and stress as possible.It is therefore argued that an open-ended capacity for learning is maximally adding to survival if paired to two distinct tendencies:1) a tendency to seek high-arousal evoking situations whenever surplus energy is available, and 2) a tendency to seek arousal reducing situations as soon as the surplus energy is exhausted.